Basically, you have to research the laws governing giveaways in whatever country you are opening your giveaway to. I have found some info on a few countries you can start with.

Countries That Require Translation Into Official Language:
Giveaway rules and in some cases related advertisements, websties, and other related materials. must be translated into the official language of these countries to be legal: Argentina, France, Mexico, and Poland. I'm not sure what other countries may share this requirement, and the countries listed above may have additional legal restrictions. (Source)

Countries That Tax Winner of Prize
India, Norway, and the United States all tax winner's prizes. Other countries may do this as well. So it's important to state that taxes are the responsibility of the winner (unless of course you plan to pay them). (Source)

Countries That Tax Prize Provider or Giveaway Host
Some countries, including Brazil, Mexico, Spain, and parts of Australia impose taxes directly on the marketer and may even require pre-registration with local tax authorities. (Source 1, 2)

THE U.S. Government Has Trade Restrictions With Some Countries

The US government also severely restricts trade with several countries, including Cuba and North Korea...so you probably could not mail your prize to these countries if you live in the US. You can find a list of countries with special restrictions here and learn more about trade restrictions here.

Lack of Adequate Postal System

Just because someone from an area can access the internet, doesn't mean that they have a viable means to receive physical mail. There are many places around the world where the postal system is shoddy or non-existent. (Cambodia is one country I've been told is hard to get mail to people in).

Basically, that leaves me thinking that truely WW (worldwide) giveaways that are also legal are a myth. World Wide usually actually means, world-wide with exceptions.

MORE INFO ON SOME SPECIFIC COUNTRIES

Canada
Canada requires that there be some aspect of skill. You can ask Canadians a 3 part math question to reach that requirement. More info here. Unfortunately there are stricter rules for Quebec, including registering and paying a fee up to 10% of the prize value. More here.

United Kingdom
Giveaways are regulated by the Gambling Act of 2005. The good news seems to be that if you're giveaway is free you are most likely OK. You just have to make sure that entries you have won't be seen as having a monetary value. This conversation helped me out. It says that according to the Gambling commission in the UK, entries via Twitter, Facebook, etc. are considered free entries.